Alternative Splicing Controls the Mechanisms of FAK Autophosphorylation

Abstract

Focal adhesion kinase (FAK) is activated following integrin engagement or stimulation of transmembrane receptors. Autophosphorylation of FAK on Tyr-397 is a critical event, allowing binding of Src family kinases and activation of signal transduction pathways. Tissue-specific alternative splicing generates several isoforms of FAK with different autophosphorylation rates. Despite its importance, the mechanisms of FAK autophosphorylation and the basis for differences between isoforms are not known. We addressed these questions using isoforms of FAK expressed in brain. Autophosphorylation of FAK⁺, which is identical to that of “standard” FAK, was intermolecular in transfected cells, although it did not involve the formation of stable multimeric complexes. Coumermycin-induced dimerization of gyrase B-FAK⁺ chimeras triggered autophosphorylation of Tyr-397. This was independent of cell adhesion but required the C terminus of the protein. In contrast, the elevated autophosphorylation of FAK^+6,7, the major neuronal splice isoform, was not accounted for by transphosphorylation. Specifically designed immune precipitate kinase assays confirmed that autophosphorylation of FAK⁺ was intermolecular, whereas autophosphorylation of FAK^+6,7 or FAK⁺⁷ was predominantly intramolecular and insensitive to the inhibitory effects of the N-terminal domain. Our results clarify the mechanisms of FAK activation and show how alternative splicing can dramatically alter the mechanism of autophosphorylation of a protein kinase.

We thank Pascal Ezan and Sylvie Clain for valuable assistance and Hervé Enslen for critical reading of the manuscript. Michael Ferrar and Leon Perlmutter are gratefully acknowledged for providing the gyrase B construct, and Janine Ragab is gratefully acknowledged for providing RPTP-β.

A.C. was the recipient of a fellowship from the French Ministry of Foreign Affairs and the French Embassy in Uruguay. The work was supported in part by grants from the European Community (Bio4 CT98 0526), the Human Frontier Science Program, the Fondation pour la Recherche Medicale, and the Fondation Schlumberger pour l'Enseignement et la Recherche (Dotation Annette Gruner Schlumberger) to J.A.G.

The first two authors contributed equally to this work.